1. Field of the Invention
The present invention relates to a grid in a nuclear fuel assembly for supporting a plurality of fuel rods, control-rod guide thimbles and so on.
2. Prior Art
FIG. 9 of the attached drawings depicts an example of a nuclear fuel assembly which is mounted on a nuclear reactor such as a pressurized water reactor or the like.
In this assembly, a pair of top and bottom nozzles 1 and 2 are arranged in vertically spaced relation to each other so as to face each other. A plurality of zircaloy guide thimbles 3 such as control-rod guide thimbles and measuring-instrument guide thimbles are extended between and securely fixed to the top and bottom nozzles 1 and 2, and a plurality of grids 4 are secured to intermediate portions of these guide thimbles 3 in vertically spaced relation to each other.
As shown in FIGS. 10 and 11, each of the grids 4 includes a plurality of straps 6 assembled perpendicularly to each other to define a plurality of grid cells 5. A pair of dimples 9 and a pair of springs 10 for supporting a fuel rod 8 are formed on the wall surfaces 6a of the straps 6 of each grid cell 5 in opposed relation to each other. Furthermore, a plurality of sleeves 11 are securely fixed to the upper and lower opening ends of prescribed grid cells 5, respectively, for guiding the outer peripheries of the thimbles 3 to be inserted into the grid cells 4.
Each sleeve 11 is of an elongated cylindrical shape having an inner diameter generally identical to the intervening distance between the opposite straps of the grid cell 5, and as shown in FIG. 12, four openings 12 are formed in an outer peripheral surface 11a thereof in circumferentially equally spaced relation to one another, whereby four wall portions 11b are defined at the central portion thereof. For securing each sleeve 11 to a prescribed grid cell 5, the sleeve 11 is first inserted into the grid cell 5 through the upper or lower opening end in such a manner that its wall portions 11b are opposed to the corner portions 5a of the grid cell 5, and that the opposite ends of the sleeve 11 protrude from the upper and lower opening ends of the grid cell 5, and is then fixedly secured to the grid cell 7 by means of brazing or the like.
In the aforesaid grid 4, when inserted into the grid cell 5, the fuel rod 8 is caused to move while its outer peripheral surface is being clamped between the dimples 9 and the springs 10. Hence, there is a problem that the outer peripheral surface of the fuel rod 8 is scratched by the springs 10 and the dimples 9 along its longitudinal direction.
Therefore, a grid cell 20 of a construction as shown in FIGS. 13 and 14 is employed. In this grid 20, each of the elongated straps 21 has a plurality of cut-outs 22 formed therein so as to be spaced a predetermined distance from each other in a longitudinal direction of the strap 21, and a spring 24 is formed between each pair of adjacent cut-outs 22 in the strap 21 so as to project into a grid cell 23. In addition, a pair of dimples 26 are formed at respective positions in the widthwise direction of the strap 21, between which the spring 24 is located in such a manner as to project in a direction opposite to the spring 24.
For inserting a plurality of fuel rods 36 into the grid cells 23 in each of the grids 20, respectively, a deflecting jig 15 as shown in FIG. 15 is employed. This deflecting jig 15 is composed of a sleeve 33 divided circumferentially into the four sleeve pieces and a tapered pin 34 inserted into the sleeve 33 for axial sliding movement, and is first inserted into the grid cell 23. Then, the tapered pin 34 is drawn into the sleeve 33 to cause the four-divided sleeve pieces to be urged and moved outwardly, so that the springs 24 are deflected by these pieces as shown in FIG. 14. Then, an elongated key member 16 is inserted into a cut-out 27 from the side of the grid 20 to bring its hook 31 into engagement with a rib 25 of the strap, and while the spring 24 is maintained deflected by the key member 30, the deflecting jig 15 is released. Thereafter, the fuel rod 36 is inserted, and the key member 30 is released. Thus, with these procedures, the fuel rod 36 is not brought into sliding contact with the springs 24 and the dimples 26 during the insertion, so that no scratches are developed on the outer peripheral surface of the fuel rod 36.
However, in the case where the grid 20 of the aforesaid construction is employed, the conventional sleeve 11 as described above cannot be utilized. This is because when the sleeve 11 is secured to the grid cell 23, the wall portion 11b of the sleeve 11 prevents the hook 31 of the key member 16 from engaging with the rib 25 of the strap 21 in the grid 20.
In order to overcome the above problems, a short sized sleeve 40 as shown in FIG. 16 may be used. This sleeve 40 is prepared by cutting out a central portion of the sleeve 11 of FIG. 12 by a width A in a circumferential direction to divide the sleeve 11 transversely into two short parts. The short sleeve 40 thus prepared can be inserted into the grid cell 23 with its four short legs 40a being opposed to the corner portions of the grid cell 23 as shown in FIG. 16, and further the hook 31 can be easily engaged with the rib 25 by inserting the key member 16 into the space of the width A in the corner portion of the grid cell 23.
However, there is still a problem that when securing the short sleeve 40 to the grid cell 23, the short sleeve 40 may be shifted from the axis of the grid cell 23, and hence the thimbles 3 cannot be arranged properly. Moreover, the surface areas that can be used for brazing the short legs 40a of the sleeve 40 to the wall surfaces of the straps 21 of the grid cell 23 are relatively small, and hence the securing strength of the short sleeve 40 with respect to the grid 20 is unduly reduced.